In recent years, transmitting not only voice data but also bulk data, like static image data and moving image data is on its way to becoming common in a cellular mobile communication system as information is increasingly transformed into multimedia. In order to bring bulk data transmission into practice, techniques for achieving a high transmission rate by utilization of a high frequency radio band have been studied actively.
However, when the high frequency radio band is utilized, a high transmission rate can be expected at a short range, whereas attenuation caused by a transmission distance becomes greater as the distance increases. Therefore, when a mobile communication system utilizing a high frequency radio band is actually put into practice, an area covered by wireless communication base station equipment (hereinafter abbreviated as a “base station”) becomes smaller. This creates need to install a larger number of base stations. Since installing the base station entails a fair cost, there exists a strong demand for a technique of implementing a communication service utilizing a high frequency radio band while preventing an increase in the number of base stations.
To meet such a demand, as represented by a wireless relay system depicted in FIG. 11, there has been examined a relayed transmission technique for installing a wireless communication relay node device 20 (hereinafter referred to as a “relay node”) between a base station 10 and a wireless communication mobile station device 30 (hereinafter referred to as a “mobile station”) and effecting communication between the base station 10 and the mobile station 30 by way of the relay node 20 in order to enhance the area covered by the base station. Use of the relay technique makes it possible even for the mobile station 30 incapable of establishing direct communication with the base station 10 to carry out communication by way of the relay node 20. Incidentally, a mobile station 31 is a mobile station that is connected directly to the base station 10.
[Explanation of a TD Relay]
By reference to FIGS. 12 and 13, a TD relay is now described. FIG. 12 shows a conceptual rendering for explaining an uplink TD relay, and FIG. 13 shows a conceptual rendering for explaining a downlink TD relay. The TD relay (also called a “half duplex relay” or a “Type 1 relay”) separates transmission from the base station to the relay node and transmission from the relay node to the mobile station by means of time division.
In the uplink shown in FIG. 12, transmission from the mobile station 30 to the relay node 20 over an access link (Access link) is carried out by means of a subframe #2 (Subframe #2). Communication from the relay node 20 to the base station 10 over a backhaul link (Backhaul link) is carried out by means of a subframe #3 (Subframe #3). Transmission from the mobile station 30 to the relay node 20 is again carried out by means of a subframe #4. Likewise, in the downlink shown in FIG. 13, transmission from the relay node 20 to the mobile station 30 over the access link (Access link) is carried out by means of the subframe #2 (Subframe #2). Communication from the base station 10 to the relay node 20 over the backhaul link (Backhaul link) is carried out by means of the subframe #3 (Subframe #3). Communication from the relay node 20 to the mobile station 30 is again carried out by means of the subframe #4.
As mentioned above, backhaul communication and Relay communication via the access link are divided along the time domain, whereby a transmission time and a receiving time of the relay node can be divided. Therefore, the relay node can relay communication without being affected by runaround crosstalk between a transmission antenna and a receiving antenna.
[Explanations of a Guard Time]
In order to switch from transmission to reception or from reception to transmission, the relay node 20, however, must set guard periods for switching an RF (Radio Frequency) circuit. FIG. 14 is a drawing for explaining the guard periods. As shown in FIG. 14, the relay node 20 sets the guard periods in a subframe when switching from transmission to reception or from reception to transmission. The guard period is presumed to be set to about 20 [μs] depending on performance of the equipment. Since the TD relay requires the guard times as mentioned above, subframes that the relay node 20 cannot transmit or receive occur.
[Instance where the TD Relay is Applied to a Synchronization System]
There are proposals for letting timings of subframes coincide with each other by synchronizing the base station 10 and the relay node 20 by means of the GPS or the like. Timings of the frames are synchronized with each other by means of the GPS, or the like, and the base station 10 and the relay node 20 concurrently transmit DL signals and receive UL signals. Since the base station 10 and the relay node 20 concurrently perform transmission as a result of being synchronized as mentioned above, synchronization is appropriate for a case where the mobile station 30 receives signals from both the base station 10 and the relay node 20.
Moreover, if synchronization between the subframe of the base station 10 and the subframe of the relay node 20 is not achieved in a TDD system, signals in the downlink (DL) channel and the uplink (UL) channel will interfere with each other. For this reason, letting timings of the subframes coincide with each other has been examined. In the TDD system, the UL and the DL are separated from each other by a time, and the same frequency is occupied by the UL and the DL.
FIG. 15 shows an example of the TDD system. The TDD system determines timing of the subframe of the DL and timing of the subframe of the UL while taking timing of a subframe of the base station 10 as a reference. The timing of the subframe of the DL corresponds to transmission timing of the base station 10. The timing of the subframe of the UL corresponds to timing at which the base station 10 receives the UL signal. The mobile station 30 receives the DL signal including a delay equivalent to a time consumed by transmission from the base station 10 to the mobile station 30. In order to let the timing at which the base station 10 is to receive the UL signal coincide with receiving timing of the mobile station 30, the mobile station 30 transmits the UL signal earlier a time equivalent to the delay due to transmission from the base station 10 to the mobile station 30.
A subframe used for switching from the DL to the UL (i.e., Subframe 2 in FIG. 15) is called a “Special Subframe,” and both the DL signal and the UL signal exist in this subframe. In the TDD system, when the subframe of the base station 10 and the subframe of the relay node 20 are not synchronized with each other, when the relay node 20 receives the UL signal from the mobile station 30 by means of the special subframe, there arises a problem of an increase in probability of the DL signal transmitted from the base station 10 to the mobile station 30 inducing interference.
[Explanations of ACK/NACK Signals Transmitted to PUCCH]
FIG. 16 shows an example signal of one slot. As shown in FIG. 16, a plurality of ACK/NACK signals transmitted from a plurality of terminals (the mobile stations 30) are spread along the time domain by means of a ZAC (Zero Autocorrelation) sequence exhibiting a zero autocorrelation characteristic, a Walsh sequence, and a DFT (Discrete Fourier Transform) sequence, and the thus-spread signals remain code-multiplexed within the PUCCH. Further, reference symbols (W0, W1, W2, W3) in FIG. 16 represent a Walsh sequence of sequence length 4, and reference symbols (F0, F1, F2) represent a DFT sequence of sequence length 3.
As shown in FIG. 16, in a terminal (the mobile station 30), an ACK or NACK answer signal is first transformed by means of a ZAC sequence (of sequence length 12) into a frequency component corresponding to 1SC-FDMA symbol along a frequency domain with use of primary spread. Subsequently, the answer signal subjected to primary spread and the ZAC sequence serving as a reference signal are subjected to secondary spread in correspondence with a Walsh sequence (of sequence length 4: W0 to W3) and a DFT sequence (of sequence length 3: F0 to F3). Moreover, the signals subjected to second spread are transformed into a signal of sequence length 12 on a time domain by means of IFFT (Inverse Fast Fourier Transform). CP is added to each of the signals subjected to IFFT, whereby one slot signal comprised of seven SC-FDMA symbols is produced.
Answer signals from different terminals (mobile stations 30) are spread by use of ZAC sequences corresponding to different quantities of cyclic shift (Cyclic shift indices) or orthogonal code sequences corresponding to different sequence numbers (Orthogonal cover indices: OC indices). The orthogonal code sequence corresponds to a set of Walsh sequence and DFT sequence. Moreover, the orthogonal code sequence is referred to also as a block-wise spreading code sequence (Block-wise spreading code). Accordingly, the base station 10 can separate a plurality of code-multiplexed answer signals apart from each other by use of existing despreading and correlation processing.
FIGS. 17A and 17B show a PUCCH arranged in a resource block (Resource Block: RB). FIG. 17A shows an example PUCCH (1) arranged in the RB, and FIG. 17B shows another example PUCCH (2) arranged in the RB. In FIGS. 17A, 17B, one subframe is comprised of two slots, and each of the slots is made up of the seven symbols shown in FIG. 16.
As shown in FIG. 17A, in order to acquire a frequency diversity gain between slots, a first slot and a second slot are transmitted to different RBs, respectively. Further, FIG. 17B shows a case where an SRS (Sounding Reference Signal) is arranged at a #13 symbol. In this case, an ACK/NACK symbol of Slot 2 becomes three symbols; hence, the DFT sequence is used also for the three symbols.